1. Introduction
This invention relates to the metal plating of plastics and more particularly, to a novel solution useful for the removal of oxidation residues in a process for plating plastics by electroless deposition where entrapment of solution in capillaries is a problem.
2. Description of the Prior Art
It is known in the art that plastics may be plated with metal for a variety of purposes. For example, two major uses of metal plated plastics are for decoration and for the fabrication of electronic devices. An example of a decorative use includes automobile parts such as trim. Examples of electronics uses include printed circuits, where the metal plate in a selective pattern comprises the conductors of the printed circuit board, and metal plated foam used for EMI shielding. ABS resins are the most commonly plated plastics for decorative purposes while phenolics and epoxies are the most commonly plated plastics for the fabrication of printed circuit boards. In general, a process for metal plating a plastic by electroless deposition may include a first step of solvent treatment followed by the steps of oxidation to render the surface of a plastic part hydrophilic and therefore adsorbtive of a plating catalyst, catalysis to render the plastic catalytic to electroless metal deposition and metal plating comprising electroless metal deposition optionally followed by electrolytic deposition to provide a decorative finish and/or increase the thickness of the metal plate.
The bond strength between the metal plate and the plastic is of prime importance to the final product whether the product is decorative or a printed circuit. If a plastic is plated by a process involving only the steps of cleaning, catalysis and plating, the resultant bond strength between the part and the metal plate will be minimal and unacceptable for the majority of commercial applications. Consequently, much research has been devoted to processes for pretreatment of plastics prior to plating, all with the object of increasing bond strength between the plastic and the metal plate. The majority of these processes include the sequential steps of solvent treatment and oxidation. An early publication disclosing a process for preparing a plastic for metal plating is Narcus, Metallizing Of Plastics, Reinhold Publishing Corporation, New York, 1960, Chapter 2. In this publication, it is taught that the plastic part is treated with an oxidizing solution of sulfuric acid and a source of hexavalent chromium ions. An early patent showing solvent pretreatment followed by a chromic acid oxidative etch is U.S. Pat. No. 3,574,070 where the first treatment step comprises solvent treatment with an aqueous emulsion of a solvent followed by treatment with a hexavalent chromium solution. Patents showing pretreatment of specific plastics include U.S. Pat. Nos. 3,445,350; 3,598,630 and 3,769,061, each for ABS; U.S. Pat. No. 4,125,649 for polysulfone; and U.S. Pat. No. 4,315,045 for polyamides.
In commercial practice, essentially all processes utilize a chromic acid solution as a source of hexavalent chromium for the step of oxidation. A typical chromic acid solution is disclosed in Example I of the above identified U.S. Pat. No. 3,445,350 incorporated herein by reference. However, it is known in the art that the use of a chromic acid etchant has serious drawbacks including the toxicity of chromium compounds which makes their disposal difficult; chromic acid residues on the plastic surface that inhibit electroless deposition; and the difficulty of rinsing chromic acid residues from the plastic surface following treatment.
Attempts have been made in the prior art to substitute other oxidants for the hexavalent chromium solutions. The most commonly proposed oxidant is a potassium permanganate solution as suggested in U.S. Pat. No. 3,625,758 where an acid permanganate solution is disclosed. A more recent attempt at such substitution is disclosed in U.S. Pat. Nos. 4,042,729 and 4,073,740 incorporated herein by reference. In these patents, instability of prior art acid permanganate solutions is identified as the problem associated with the use of such solutions and in accordance with the teachings of said patents, solution stability is achieved by use of a permanganate solution where the molar ratio of manganate ion to permanganate ion is not allowed to exceed 1.2 while the pH of the solution is adjusted between 11 and 13. An improvement in the use of permanganate solutions is disclosed in commonly assigned, copending U.S. patent application Ser. No. 542,036 filed Oct. 14, 1983.
Regardless of whether the oxidant solution is a hexavalent chromium solution or a permanganate solution, contact with the solution leaves an oxidant residue on the surface of the plastic part that acts to poison the catalytic surface interfering with metal deposition and often resulting in void formation. Using either a hexavalent chromium solution or a permanganate solution, a simple water rinse is inadequate to remove the residue and the art has resorted to a further step of contact with a solution of a reducing agent though more chemistry is involved in removal than simple reduction. Removal of permanganate residue with a reducing agent is disclosed in the above referenced U.S. Pat. Nos. 4,042,729 and 4,073,743.
When capillary geometries are on or adjacent to the plastic part to be plated, such as the micropores found in plastic foam, the problem of oxidant poisoning is increased. Capillary forces draw and retain oxidant solution and residue into the pores resulting in a high concentration of both retained in the foam following contact with the oxidant solution. These materials diffuse from the capillaries during subsequent plating steps resulting in areas surrounding the capillaries contaminated with oxidant solution and residue that inhibits plating. This results in the plated part having unplated areas (voids) over its surface where diffusion of oxidant and residue have taken place. It should be noted that a foamed plastic generally has a non-porous layer over its surface as a consequence of the molding operation during the formation of the foamed plastic. Therefore, it would be expected that the pores would not be adsorbent of the plating chemicals. However, it has been found that the non-porous surface layer is inadequate to withstand the treatment chemicals and the solutions do attack this layer resulting in diffusion of the chemicals into the subsurface pores and the resultant problems described above are encountered in the plating of foam. The demand for metal plated plastic foam is increasing because of the use of such materials as a shielding to prevent EMI leakage from electronic equipment. Consequently, the art is searching for an improved method for removing oxidant solutions and residue from the surface of a plastic part following treatment with an oxidant solution.
Another problem in the art associated with the entrapment and subsequent diffusion of oxidant and residue from capillaries is associated with the racks used to carry the plastic part through the plating sequence. These racks are often metal that is coated with a plastisol that resists plating. The ends of the racks are stripped of the plastisol coating to expose the metal and hold the plastic part on the rack. The exposed metal acts as the cathodic contact for a subsequent electroplating step. In this way, following electroless metal plating, the part may be electroplated without reracking because the metal contact holding the part acts as the electrode. The difficulty associated with the use of these racks is similar to that with foam. Capillary action draws oxidant solution into the interstices between the exposed metal and its plastisol coating. During the plating sequence, oxidant solution and residue diffuse from these interstices poisoning the plastic surface adjacent the point of contact with the rack. This often results in the formation of a void for the same reasons as described above. The void acts as an electrical barrier between the rack at its point of contact with the plastic part and the metal plate that interferes with electroplating of the part or causes other related difficulties. Void formation may result in rejection of from about 1 to 15 percent of the plated parts where racks and contacts are used as described.